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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/91133] [8/9/10/11 Regression] Wrong "partial specialization
 is not more specialized than" error
Date: Fri, 01 May 2020 17:15:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D91133

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
This is really a question of partial ordering; determining whether the part=
ial
specialization is more specialized than the primary class template is
equivalent to this testcase:

template<typename T> struct Id { typedef T type; };
template<typename T, typename U, U X> struct A {};

template<typename T, typename U, U X>        void f(A<T, U,   X>); // #1
template<typename T, typename Id<T>::type X> void f(A<T, int, X>); // #2

int main()
{
  f(A<int,int,42>()); // is #2 more specialized?
}

This was rejected as ambiguous by GCC going back at least to 4.1.  It is al=
so
rejected by EDG/icc.  It is accepted by clang and msvc, like the original
testcase.

The issue is with the partial ordering deduction of #1 from #2: we deduce i=
nt
for U from the second argument, and Id<T>::type for U from the third argume=
nt,
and those don't agree, so deduction for the third argument fails in both
directions, and the functions are ambiguous.

This is related to open core issues 455 and 1337.

I don't know what rationale clang/msvc are using to conclude that #2 is more
specialized.=